Screw extruder with improved dispersive mixing

ABSTRACT

A screw extruder (10), including a barrel (18) having a bore (26) which defines an inner surface (24). At least one extruder screw (28) is positioned within the bore (26), each screw (28) further including a central shaft (34) and at least one screw flight (30). Each screw flight (30) further includes a front pushing face (36) and a rear face (38). The profile (40) of each front pushing face (36) acts with the inner surface (24) of the barrel (18) to form a progressively narrowing passage (46) through which material is forced into multiple regions of high elongational and shear stress (48) which act to break down agglomerates of material and thus dispersively mix the material. 
     Also presented is a screw extruder (100) having a single screw (28) which includes dispersion disks (102) which are rotationally offset from one another and whose design is not constrained by interaction with a second disk, which provides improved distributive and dispersive mixing. 
     Also presented is a method of extruding material using a screw extruder (10) having pushing face (36) profiles (40) which form progressively narrowing passages (46) in conjunction with the barrel (18) inner surfaces (24) to provide improved dispersive mixing.

TECHNICAL FIELD

The present invention relates generally to machines for extrusion ofmaterials and more particularly to screw extruders adapted for use withplastics and plastic-like materials. The inventor anticipates thatprimary application of the present invention will be for the manufactureof color concentrates, polymer blends, and polymer alloys.

BACKGROUND ART

A screw extruder is a machine in which material, usually some form ofplastic, is forced under pressure to flow through a contoured orifice inorder to shape the material. Screw extruders are generally composed of ahousing, which is usually a cylindrical barrel section, surrounding acentral motor-driven screw. At a first end of the barrel is a feedhousing containing a feed opening through which new material, usuallyplastic particles, is introduced into the barrel. The screw containsraised portions called flights having a larger radial diameter than thescrew's central shaft and which are usually wrapped in a helical mannerabout the central shaft. The material is then conveyed by these screwflights toward the second end of the barrel through a melting zone,where the material is heated under carefully controlled conditions tomelt the material, and then passes through a melt-conveying zone, alsocalled a pumping zone. The melted plastic is finally pressed through ashaped opening or die to form the extrudate.

Besides conveying material toward the die for extrusion, the screw isdepended upon to perform mixing of the feed material. Very generally,mixing can be defined as a process to reduce the non-uniformity of acomposition. The basic mechanism involved is to induce physical motionin the ingredients. The two types of mixing that are important in screwextruder operation are distribution and dispersion. Distributive mixingis used for the purpose of increasing the randomness of the spatialdistribution of the particles without reducing the size of theseparticles. Dispersive mixing refers to processes that reduce the size ofcohesive particles as well as randomizing their positions. In dispersivemixing, solid components, such as agglomerates, or high viscositydroplets are exposed to sufficiently high stresses to cause them toexceed their yield stress, and they are thus broken down into smallerparticles. The size and shape of the agglomerates and the nature of thebonds holding the agglomerate together will determine the amount ofstress required to break up the agglomerates. The applied stress caneither be shear stress or elongational stress and generally,elongational stress is more efficient in achieving dispersion than isshear stress. An example of dispersive mixing is the manufacture of acolor concentrate where the breakdown of pigment agglomerates below acertain critical size is crucial. An example of distributive mixing isthe manufacture of miscible polymer blends, where the viscosities of thecomponents are reasonably close together. Thus, in dispersive mixing,there will always be distributive mixing, but distributive mixing willnot always produce dispersive mixing.

In extrusion processes, the need for good dispersive mixing is oftenmore important than for distributive mixing. This is particularly truein the extrusion of compounds which contain pigments which must beuniformly mixed or small gage extrusion such as spinning of fibers orextrusion of thin films.

In screw extruders, significant mixing occurs only after the polymer hasmelted. Thus, the mixing zone is thought of as extending from the startof the melting zone to the end of the extrusion die. Within this areathere will be considerable non-uniformities in the intensity of themixing action and the duration of the mixing action, both in the barrelsection and in the extrusion die. In molten polymer, the stress isdetermined by the product of the polymer melt viscosity and rate ofdeformation. Therefore, in general, dispersive mixing should be done atas low a temperature as possible to increase the viscosity of the fluid,and with it, the stresses in the polymer melt.

Fluid elements are spoken of as having a "mixing history", which refersto the amount of elongational and shear stress to which it has beenexposed, and the duration of that exposure. A polymer that melts earlyin the mixing zone process will have a more significant mixing historythan one that melts near the end of the melting zone.

Generally, in an extruder with a simple conveying screw the level ofstress or the fraction of the fluid exposed to it is not high enough toachieve good dispersive mixing. Distributive mixing is easier to achievethan dispersive mixing, but unmodified screws have also been found toproduce inadequate distributive mixing for many applications. Therefore,numerous variations in screw design have been attempted in priorinventions to increase the amount of distributive or dispersive mixingin screw extruders. These devices usually contain a standard screwsection near the material input hopper, and one or more speciallydesigned sections to enhance mixing. These mixing sections naturallyfall into the categories of distributive and dispersive mixing elements.

Varieties of distributive mixing elements are shown in FIG. 2 A-F.Practically any disruption of the velocity profiles in the screw channelwill cause distributive mixing. Thus even simple devices, such as theplacement of pins (see FIG. 2A) between the screw flights can enhancedistributive mixing. FIG. 2B shows the well-known Dulmage mixingsection, in which the polymer flow is divided into many narrow channels,which are combined and divided again several times. The Saxton mixingsection (FIG. 2C) and the "pineapple" mixing section (FIG. 2D) are usedto produce similar results. FIG. 2E shows a screw which has slots cutinto the flights. A variation called the Cavity Transfer Mixer is shownin FIG. 2F. There are cavities both in the rotor and the barrel. Thistype of device reportedly performs both dispersive and distributivemixing.

In addition to these devices, static mixers are often used to divide andrecombine the melt stream to intermingle the material and eliminatevariations in temperature, composition and mixing history. Thesegenerally do not provide regions of high stress, and are thus mostlyused for distributive mixing.

The devices shown in FIG. 2A-F have been primarily classified asdistributive mixers because their action is mainly to spatiallyredistribute material without subjecting it to regions of high shearstress. The variations shown in FIG. 2G-J are designed to include highshear stress regions and thus perform dispersive mixing.

The most common dispersive mixing section is the fluted or splinedmixing section in which one or more barrier flights are placed along thescrew so that material has to flow over them. In passing through thebarrier clearance, the material is subjected to a high shear rate whichacts to break up agglomerates. One such device is the Maddock mixingsection, which is shown in FIG. 2G. The Maddock has longitudinal splinesthat form a set of semicircular grooves. Alternate grooves are open onthe upstream and downstream ends. Material that enters the inlet groovesis forced to pass over the mixing flights, which are shown as crosshatched areas, before reaching the outlet grooves. While passing overthe mixing flights, the material is subjected to high shear stress. Thedisadvantage of this type of mixing element is that it reduces thepressure at the output side of the mixing section and thus reduces theoutput of the extruder. Also there may be regions in which material maystagnate since the grooves have constant depth in a longitudinaldirection. This makes it less suitable for materials of limited thermalstability.

FIG. 2H shows the Egan mixing section, which has splines that run in ahelical direction to form channels separated by mixing barriers. Thesechannels can have a gradually reducing depth, tapering to zero depth atthe end of the mixing section, which reduces the chance of stagnationpoints. This helical design consumes less pressure than the Maddockstyle, thus producing less reduction in extruder output.

A blister ring, shown in FIG. 2J, is simply a cylindrical section on thescrew that has a small radial clearance, through which all material mustpass. This can cause a large pressure drop on the output side of theblister ring, resulting in a significant reduction in overall extruderoutput.

Screw extruders can have more than one central screw. Twin-screwextruders may operate with two screws that may either rotate in the samedirection, or they may be counter-rotating. There are some machines thatuse more than two screws.

In counter-rotating twin-screw extruders, dispersive mixing primarilyoccurs in the intermeshing region between the screws. This action issimilar to that in a two-roll mill. This configuration has thedisadvantage that the mixing action creates substantial separatingforces on the screws. These forces can push the screws against thebarrel, if these forces grow too great. This can cause wear on thescrews and the barrel, thus the screw speed has to be kept low, withresulting decrease in the throughput of the extruder.

In intermeshing co-rotating twin-screw extruders, the screw surfaces inthe intermeshing region move in opposite directions. As a result, mostof the material bypasses the intermeshing region and moves from onescrew to the other repeatedly.

Some twin screw machines have kneading blocks included to increasedispersive mixing. These kneading blocks are most commonly flat paddlesof roughly elliptical shape which are stacked on a central shaft, butoffset at varying angles. Each paddle on the shaft is paired with acorresponding paddle on the second shaft. The shafts usually both rotatein the same direction but with the angular orientation of the paddlesstaggered at a certain angle. We can consider the elliptical paddleshapes to have a major and a minor axis with a "tip" on each end of themajor axis and a "mid-point" at each end of the minor axis. At one pointin the rotation cycle, a tip of a paddle on the first shaft, whenhorizontally oriented, will nearly contact the midpoint of a paddle onthe second shaft, whose tip will then be vertical. As this second,vertical tip rotates towards horizontal, the first tip traces along theelliptical outline of the second paddle, thus "wiping" it. At a furtherpoint in the rotation cycle, the second paddle wipes the outline of thefirst. This wiping action keeps material from stagnating or collectingon the paddle edges. It also imposes constraints on the shapes of thepaddles, as the travel of the tip of the neighboring paddle defines theoutline of the paddle itself. Although this configuration of paddles canproduce fairly good elongational stress in material, the aboveconstraint on the shape of the paddles prevents variations in design,which may produce even better elongational stress regions.

In general, twin-screw extruders are considered to be better atdispersive mixing than single-screw extruders. However for a givencapacity, multi-screw machines are usually considerably more costly thansingle-screw extruders.

For improved mixing to occur, there are several important aspects to beconsidered. In dispersive mixing, it is the passage of material througha region of high stress that produces the desired breakdown ofagglomerates. A single pass through a high stress region will likelyachieve only a single rupture of the agglomerate. To achieve a finescale of dispersion, multiple passes and ruptures may be necessary.Also, for efficient dispersive mixing, stresses in the high stressregion should have a strong elongational component, as well as a shearcomponent. For efficient operation of the extruder as a whole, a lowpressure drop across the mixing section is desirable. It is alsoimportant to combine dispersive and distributive mixing to achieve amore uniform overall mixture. Some distributive mixing occurs wheneverdispersive mixing is done, but by deliberately combining distributiveelements with the dispersive elements, chances are improved that allfluid elements will pass through the high stress region, preferably manytimes, for proper dispersion.

To make sure that all agglomerates and droplets pass through high stressregions at least once, the flow rate through the high stress regionsmust be large enough compared to the overall forward flow rate. This canbe done by designing the number of high stress regions, their length andthe size of the gap through which material will pass. It is alsopreferable that there be more than one high stress region, and thatthese regions are symmetrically arranged around the circumference of anysection along the length of the screw, so that forces will be balancedand the possibility of deflection of the screw will be minimized. Toreduce pressure drop in the mixing section, it is desirable to have thehigh stress regions in a forward helical orientation, which can be doneby a continuous forward helix or in a stepped forward helix withkneading disks.

For the foregoing reasons, there is a great need for a screw extruderwhich provides better dispersive mixing than in presently availableextruders.

DISCLOSURE OF INVENTION

Accordingly, it is an object of the present invention to provide a screwextruder which provides improved dispersive and distributive mixing.

Another object of the present invention is to provide a screw extruderin which material is repeatedly passed through regions of high stressfor better break-down of agglomerates into smaller particles.

And, another object of the invention is to provide a screw extruderhaving a number of high stress regions which produce high elongationalstress as well as shear stress.

Yet another object of the invention is to provide a single screwextruder which is less costly to manufacture for its capacity than amulti-screw extruder, but which provides dispersive mixing comparable orbetter than conventional multi-screw extruders.

A further object of the present invention is to provide a screw extruderin which the high stress regions do not produce a large pressure dropwhich may impede overall throughput.

A still further object of the present invention is to provide a screwextruder that has symmetrically balanced high stress regions which avoidpossible deflection of the central screw.

A yet further object of the present invention is to provide amulti-screw extruder that provides improved dispersive mixing overpresent multi-screw extruders.

An additional object is to provide a screw extruder with modular,exchangeable mixing sections.

A further additional object is to provide a single screw extruder withkneading paddles whose design geometry is not constrained as in twinscrew extruders, and which provides improved dispersive and distributivemixing.

Briefly, one preferred embodiment of the present invention is a screwextruder including a barrel having a bore that defines an inner surface.At least one extruder screw is positioned within the bore, each screwfurther including a central shaft and at least one screw flight. Eachscrew flight further includes a front pushing face and a rear face. Theprofile of each front pushing face acts with the inner surface of thebarrel to form a progressively narrowing passage through which materialis forced into multiple regions of high elongational and shear stresswhich act to break down agglomerates of material and thus dispersivelymix the material.

A second preferred embodiment of the present invention is a screwextruder having a single screw which includes dispersion disks which arerotationally offset from one another and whose design is not constrainedby interaction with a second disk to provide improved distributive anddispersive mixing. There may also be transition disks between thedispersion disks.

A third preferred embodiment of the present invention is a screwextruder having kneading paddles which include at least one dispersiongroove which channels material into regions of high stress.

A fourth preferred embodiment of the present invention is a screwextruder having at least one extruder screw that has at least one flutedmixing section. Each mixing section further has inlet and outletsections which are separated by a plurality of barrier flights which actto force material through regions of elongational and shear stress.

A fifth preferred embodiment of the present invention is a screwextruder having defined a flight clearance δ a channel depth H, a helixangle φ, a mixing section length L, a screw diameter D, a flight tipwidth w_(f), a channel width, where for a range of δ/H of 0.05-0.5,mixing section length L lies in the range 1-20 times the screw diameterD, the helix angle φ lies in the range from -90° to -30° and +30° to+90°, the flight tip width w_(f), lies in the range 0-0.5 times thechannel width and the δ/D ratio lies in the range of 0.005-0.250.

Also presented is a method of extruding material using a screw extruderhaving pushing face profiles that form progressively narrowing passagesin conjunction with barrel inner surfaces to provide improved dispersivemixing.

The described versions of the present invention have many advantageswhich address the above-mentioned objects. One such advantage of thepresent invention is that provides both good dispersive and gooddistributive mixing.

Another advantage of the invention is that it provides regions of bothhigh elongational and shear stress.

A further advantage of the present invention is that material isrepeatedly passed through regions of high stress for improved breakdownof material agglomerates.

Yet another advantage of the invention is that it may be used in asingle-screw extruder, which is less costly for its capacity than acomparable twin screw extruder, yet may provide comparable or betterdispersive mixing.

A still further advantage of the present invention is that the highstress regions may be in a forward helical orientation, thus reducingpressure drop in the mixing section, and therefore enhancing overallefficiency and throughput.

A yet further advantage of the present invention is that the high stressregions are symmetrically arranged around the central screw, so thatdeflection is minimized.

These and other objects and advantages of the present invention willbecome clear to those skilled in the art in view of the description ofthe best presently known mode of carrying out the invention and theindustrial applicability of the preferred embodiment as described hereinand as illustrated in the several figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The purposes and advantages of the present invention will be apparentfrom the following detailed description in conjunction with the appendeddrawings in which:

FIG. 1 is an overhead view of a simplified screw extruder, in which aportion of the barrel has been cut-away;

FIGS. 2A-J illustrates a variety of prior art mixing sections fordistributive and dispersive mixing;

FIG. 3 shows an isometric view of an extruder screw according to oneembodiment of the present invention;

FIG. 4 illustrates a cross-sectional view taken through the screwextruder along line A--A in FIG. 1;

FIGS. 5A-F shows a variety of flight face profiles which embody thepresent invention;

FIG. 6 illustrates a plan view of a prior art mixing sectioncorresponding to a detailed view of the prior art mixing section seenabove in FIG. 2G;

FIG. 7 shows a plan view of an improved mixing section according to anembodiment of the present invention;

FIG. 8 is a side view of a screw extruder with a portion of the barrelremoved to expose an extruder screw which has been formed according tothe present invention;

FIG. 9 is a cross-sectional view taken through the screw extruder alongline B--B in FIG. 8;

FIG. 10 shows a perspective view of prior art kneading paddles;

FIG. 11 illustrates a perspective view of dispersion disks, which are apreferred embodiment of the present invention;

FIG. 12 shows a perspective view of a screw extruder of which a portionof the barrel has been cut away, including dispersion disks of thepresent invention;

FIG. 13 illustrates a side view of prior art kneading paddles includingapproach angle α;

FIG. 14 shows a side view of the dispersion disks of the presentinvention, including approach angle α;

FIGS. 15A & B show front and side plan views of a dispersion disk of thepresent invention;

FIGS. 16A & B show front and side plan views of a further embodiment ofa dispersion disk of the present invention;

FIGS. 17A & B illustrate front and side plan views of a prior artkneading paddle;

FIGS. 18A & B show front and side plan views of a modified kneadingpaddle according to the present invention; and

FIG. 19 shows a side plan view of a stack of dispersion disks andinterleaved transitional elements according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention is a screw extruderhaving improved dispersive mixing. As illustrated in the variousdrawings herein, and particularly in the view of FIG. 1, a form of thispreferred embodiment of the inventive device is depicted by the generalreference character 10.

FIG. 1 illustrates the major portions of a screw extruder 10, having acentral longitudinal axis 11, and which also has an input end 12 and anoutput end 14.

Generally, for convenience of reference, the terms "downstream" shallrefer to those ends closest to the output portion of the screw extruderand the term "upstream" shall refer to those ends farthest away from theoutput. The downstream direction is indicated by a large arrow, whichshows the direction of material flow. The screw extruder 10 has a barrel18. The input end 12 includes an input hopper 20 for feeding inmaterial, and an extrusion die 22 on the output end 14. A portion of thebarrel 18 has been cut away to show the barrel wall 24, and an innerbore 26. Positioned within the bore 26 is an extrusion screw 28 havingscrew flights 30. Although this version of the preferred embodiment hasa single screw, it is to be understood that the screw extruder couldcontain two or more screws.

FIG. 2 illustrates a variety of prior art mixing sections of extruderscrews, as discussed above in the Background Art section.

FIG. 3 illustrates an isometric cross-sectional view of one version of amixing section of an extruder screw 28 used in the present invention.The general reference character 31 designates the mixing section of theextruder screw 28. This mixing section can be an integral portion of theoverall screw 28, or it may be a modular section which is combined withother modular sections upon a central shaft to provide varyingcharacteristics which can be customized for different materials andapplications. In FIG. 3, an optional central bore 32 is shown in phantomline for the case where mixing section 31 is a modular section which canbe stacked upon an independent central shaft. This mixing section isdefined as having a length (L) 33. The mixing section 32 includes acentral shaft 34 and screw flights 30, which are used to mix thematerial as well as convey it forward. A screw diameter (D) 35 isdefined as the tip to tip distance between flights 30 when positioned onopposite sides of the central shaft 34. An arrow is included to indicatethe direction of rotation. The flights 30 include the forward pushingface 36, and the rear face 38. Reference character 40 will be used torefer to the cross-sectional profile of the flight 30. As will be seenbelow, it is the shape of this profile 40 that produces the multipleregions of high elongational and shear stress, which are crucial inproducing increased dispersive mixing. In this version of the preferredembodiment, slots 42 have been provided to increase distributive mixing.These slots 42 are optional, and their size and placement are subject toconsiderable variation, as will be apparent to one skilled in the art.Additionally many other distributive mixing devices may be used,including, but not limited to, pins or protrusions which could be round,square, diamond shape or irregular, secondary flights withinterruptions, and changes in channel depth or width (defined below).

FIG. 4 shows a cross-sectional view through the screw extruder 10 alongline A--A in FIG. 1. Barrel 18 includes the barrel wall 24 surrounding acentral bore 26 which is defined by the barrel inner surface 44. In thisversion of the preferred embodiment, the central bore 26 is cylindricaland accommodates a single extruder screw 28, but it should be understoodthat the present invention can be used with two or more extruder screws,in which case, the bore 26 cross-section may resemble multipleoverlapping circles or ellipses.

The flights 30 of the screw 28 are shown, and in this version of thepreferred embodiment there are four flights which are positioned at 90°degree intervals around the circumference of the central shaft 34. It isto be understood that other numbers of flights such as two, three, five,six, etc. may be used, and their positions around the circumference ofthe shaft 34 is likewise variable. It is desirable, however, that theflights 30 be symmetrically arranged around the shaft 34 circumferencein order that the forces on the shaft 34 are balanced and deflection isminimized. The front pushing faces 36 and the rear faces 38 of theflights 30 are shown, as well as the flight profile 40. The frontpushing faces 36 and the inner surface of the barrel 44 define aprogressively narrowing passage 46 into which the material in the bore26 moves. The rotational movement of the screw 28 causes the material toenter the large end of the passage 46 and to be squeezed as the passage46 narrows into high stress regions 48, which are created between theflight tips 50 and the barrel inner surface 44. The reducing crosssectional area of the progressively narrowing passage 46 causes anincrease in the average flow velocity. Thus the high stress regions 48develop both elongational and shear stresses which are important inproviding increased dispersive mixing. The action of the screw 28rotation ensures that material agglomerates passes through many of thesehigh stress regions 48, and thus very high quality dispersive mixing maybe obtained.

The distance between the barrel inner bore 44 and the central shaft 34will define the channel height (H) 52. The distance between the flighttip 50 and the barrel inner bore 44 is defined as the radial flightclearance (δ) 54. The ratio of the flight clearance 54 to the channelheight 52 is defined as δ/H. The distance between the forward pushingface 36 of one flight 30 and the rear face 38 of the next flight 30 onthe extruder screw measured circumferentially will be referred to as thechannel width 56. Additionally, the width of the tip of the flight(w_(f)) will be designated as 58, although it is quite possible thatthis w_(f) 58 may be zero, depending on the flight face profile 40.Indeed, the profile shown in FIG. 4 is a smooth curve at the tip 50 andthus the flight tip width 58 is zero in this case.

FIG. 5 shows a variety of pushing face profiles 40 of the screw flights30 which can be used in conjunction with the barrel wall 24 to produceprogressively narrowing passages 46 to force material into high stressregions 48. It should be understood that many other variations inprofile are possible, and the present invention is not limited to theprofiles shown.

The versions of the preferred embodiment discussed so far have had a"positive helix angle" or forward flighted configuration. In a forwardflighted screw, the pushing flight face moves the material forwardtoward the output end of the extruder. Returning to FIG. 1, a positivehelix angle (φ) 60 can be seen as measured between a perpendicular tothe flight face 62 and the longitudinal axis 11. It is also possible tohave a rearward flighted configuration. In this case, the pushing flightface moves the material backward toward the feed opening of theextruder. It is also possible to have a neutral flight configuration. Inthis case, the flight angle is ninety degrees, and the pushing flightface moves the material only in the circumferential direction.

In a screw extruder 10, it is possible to have a mixing section thatcontains a negative helix angle. The sections of the screw which areupstream from the mixing section may still have a positive helix anglein order to make sure that the material is conveyed toward the outputend 14 effectively. Thus it should be understood that the presentinvention 10 may be practiced with variations which include negativehelix angles. It is anticipated that the extruder screw 28 may be madeto be "modular", having a central shaft upon which mixing sections ofvarying geometry may be stacked to be adaptable to various materials andoperations.

One type of modular mixing section which may be employed is the Maddockmixing section, which was discussed in the background art section andshown in FIG. 2G. FIG. 6 PRIOR ART shows a Maddock mixing section inmore detail. The Maddock has longitudinal splines that form a set ofsemicircular grooves. Alternate grooves are open on the upstream anddownstream ends. Material that enters the inlet grooves 70 is forced topass over the mixing barrier flights 72, which are shown as crosshatched areas, before reaching the outlet grooves 74. While passing overthe mixing flights 72, the material is subjected to high shear stress.This can produce some dispersive mixing, but this prior art mixingsection only subjects material to a single pass through the high stressregion.

In contrast, FIG. 7 shows another preferred embodiment of the presentinvention in which a multi-flight mixing section 80 can be incorporatedin a modular fashion to a screw extruder. Material is introduced throughthe inlet grooves 70 as before, but must pass over a series of mixingbarrier flights 82 before reaching the outlet grooves 74. In passingover this series of flights 82, the material must repeatedly passthrough high stress regions. Additionally, the barrier flights 82 caninclude a number of progressively narrowing passages 46 to produceregions of high elongational stress 48, thus achieving much betterdispersive mixing.

It should be understood that the use of multiple mixing barriers can beused in many different configurations to improve dispersive mixing fromthat achieved by the prior art. A further example would be to improvethe blister ring mixer seen in FIG. 2J of the prior art by using aseries of small rings with wedge-shaped leading edges to create multiplehigh stress regions. Additionally, similar modifications can be made toall fluted mixers such as the Egan, (FIG. 2H), the Zorro, the Troester,etc.

In designing extruder screw mixing sections for good dispersive mixing,there are several variables which can be adjusted to optimize results.According to Tadmor and Manas-Zloczower, (Z. Tadmor and I.Manas-Zloczower, Advances in Polymer Technology, V.3, No.3 , 213-221(1983)), the passage distribution function can be written as: ##EQU1##where k is the number of passes through the clearance, the dimensionlesstime λ=t_(r) /t is the ratio of the residence time t_(r) and the meanresidence time of the controlled volume t. The residence time for aNewtonian fluid can be approximated as follows: ##EQU2## where z is thehelical length of the screw section considered, v_(bz) the down-channelbarrel velocity, and r the throttle ratio (pressure flow rate divided bydrag flow rate). The mean residence time can be determined from:##EQU3## where W is the channel width, H the channel depth, δ the radialflight clearance, and w_(f) the flight width. The dimensionless time canbe written as: ##EQU4## where L is the axial length corresponding todown-channel distance z. The fraction of the fluid experiencing zeropasses through the clearance is:

    G.sub.0 =e.sup.-λ

The G₀ fraction should be low to make sure that most of the fluidexperiences at least one or more passes through the clearance. In asimple conveying screw the G₀ fraction is usually around 0.99, whichmeans that most of the fluid passes through the extruder without everpassing through the clearance. The expressions above can be used todetermine the minimum value that will yield a G₀ less than 0.01, meaningthat less than one percent of the fluid will not pass through theclearance at all. This is achieved when the dimensionless time λ>4.6.For certain values of L, H, W, r, φ, w_(f) we can then determine howlarge the flight clearance δ has to be to make λ>4.6 or G₀ <0.01.

When L=3W, r=0, and φ=17.67°, the ratio of δ/H has to be about 0.8 toachieve a G₀ <0.01. Clearly, with such a high ratio of δ/H it will bealmost impossible to create large stresses in the clearance and toaccomplish effective dispersive mixing. From equation 4 it is clear whatgeometrical variables we have to change to achieve a low G₀ fraction ata small clearance. We can do this by 1) increasing L, the length of themixing section, 2) increasing φ, the helix angle, and 3) reducing w_(f),the width of the flight.

If the helix angle is increased from 17.67 to 60 degrees with the othervalues being the same, the δ/H ratio has to be about 0.35 or greater forthe G₀ fraction to be less than 0.01. This value is still rather larger,but substantially better than 0.8. The δ/H ratio can be further reducedby increasing the length of the mixing section or reducing the flightwidth or by increasing the helix angle even more. This procedure allowsa first order determination of the design variables. Further refinementof the initial values can be obtained from computer simulation.

It is estimated that in prior art screw extruders with a simpleconveying screw, G₀, the fraction of material that does not pass throughthe clearance, is typically 0.99. This means that a great deal ofmaterial never passes through high stress regions, and thus dispersivemixing is poor.

In the present invention, it is desired that G₀ is less than 0.01, andtherefore greater than 99% of the material will pass through a region ofhigh stress. It is anticipated from the above formulas that for thisvalue of G₀, in order to achieve a δ/H ratio in a reasonable range of0.05 to 0.3, the helix angle (φ) will lie in the range of -90° to -30°and +30° to +90°, the length of the mixing section (L) will lie in therange of 1 to 20 times the bore diameter, and the width of the screwflight at the tip (w_(f)) will lie in the range of 0 to 0.5 times thewidth of the channel.

It should be understood that although the preferred range of helixangles is -90° to -30° and +30° to +90°, values in the range from +30°to -30° will work as well.

Multiple passes through a high stress region are necessary to break downagglomerates. Thus even in mixing sections with very low values for G₀,there is no assurance that good dispersive mixing will be achieved. Forexample, in the Maddock prior art mixer, all the material has to passthrough the high stress region, but it only passes through this regiononce, which is insufficient.

Another parameter of interest is the ratio of flight clearance to thescrew diameter or δ/D. This gives an indicator of how much materialflows through the high stress regions, and thus is proportional to howwell the material is dispersively mixed. The higher this number is, thebetter the dispersion can be expected. For prior art single screwextruders, this δ/D figure is typically 0.001, and for prior art twinscrew extruders, this δ/D figure improves to 0.005. In contrast, the δ/Dvalue for the various preferred embodiments of the present invention is0.005 to 0.250. When the δ/D number becomes too high the stresses thatwill be generated may be too low to accomplish dispersive mixing. Theclearance should be small enough to be able to generate high enoughstresses but large enough to allow sufficient material to flow throughit.

The flights of a screw extruder can convey material in the downstreamdirection and can also serve to prevent material from building up on thewalls of the extruder barrel. This latter f unction is known as "wiping"and it is possible to have elements that perform this wiping separatelyfrom the function of conveying material, although typically thefunctions are combined. It is also possible that wiping and conveyingcan be performed by one set of screw flights, while mixing is done by asecond set of flights. This second set of flights may be in a separatemodular section, or in the same section by incorporating this second setof flights between the conveying/wiping flights. The mixing flights donot have to have the same helix angle as the conveying/wiping flights,and indeed it may be beneficial that they be different. The helix angleof the conveying flights can be chosen to give the best pumping action,while the helix angle for the mixing flights can be chosen to give thebest dispersive mixing action. A functional separation of the two typesof flights can thus be achieved, allowing optimization of the overallperformance.

FIG. 8 show a screw extruder 90 with a modified central screw 92 inwhich the mixing flights 94 are included between the conveying/wipingflights 96.

FIG. 9 is a cross-sectional view of FIG. 8 taken through line B--B. Theprofile 40 of the mixing flights 94 can be seen to produce aprogressively narrowing passage 46 in the barrel 18 which again forcesmaterial into regions of high stress 48. This configuration causesenhanced material dispersion while the wiping/conveying flight 96 hasbeen designed for optimal pumping characteristics.

As mentioned above, it is also possible to have a variation in which thehelix angle is ninety degrees. This is accomplished by using straightdisks which are offset from each other at varying rotational angles. Fora single screw extruder, this version of the preferred embodiment issimilar to kneading paddles used in twin screw extruders, but with theimportant difference and advantage that the geometry of the disks can beoptimized for production of effective elongational and shear stress,rather than being constrained by the need to trace the outline of asecond screw.

FIG. 10 Prior Art shows the configuration of the kneading paddles in onescrew of a twin screw extruder as in the prior art. In contrast, FIG. 11illustrates a second preferred embodiment 100 of the present inventionfor use in a single screw extruder having dispersion disks 102 alignedupon a central longitudinal axis 104. The pushing flight faces 106 havea pronounced wedge shape, and the trailing flight face 108 is flat.Stagger angle β 109 is shows the rotational offset of the successivedisks. In a similar manner to that discussed above in regard to theprofile of the pushing flight faces 30 in the positive helix angleembodiment, the profiles of the pushing faces 106 of the dispersiondisks 102 are capable of much variation, such as elliptical sections,triangles, circular sections, etc. Additionally, these dispersion disks102 may also be modular, and may be stacked upon a central shaft toprovide various mixing characteristics.

FIG. 12 shows such a version of the preferred embodiment 100 in whichdispersion disks 102 having a ninety degree helix angle have beenfixedly stacked upon a central shaft 110. A portion of the barrel 118has been cut away to show the orientation of the disks 102. A centrallongitudinal axis 104 has again been included for easy reference.Elongational and shear stress necessary for good dispersive mixing areproduced in the high stress regions 120 between the paddle tips 122 andthe barrel inner surface 124. The main dispersive mixing action willoccur in the narrowing region formed by the pushing flight face 106 andthe inner barrel surface 124. Material is forced into the progressivelynarrowing passage 126 by drag flow caused by the relative motion betweenthe screw and the barrel surface 124.

FIG. 13 PRIOR ART and FIG. 14 show a comparison of the profiles of theprior art and the dispersion disks of the present invention. In FIG. 13,a prior art kneading paddle in a barrel of a screw extruder isillustrated with line 128 drawn tangent to the surface of the paddle atthe tip. A second line 130 is drawn tangent to the point on the barrel'sinner surface which intersects the projection of line 128. The two lines128 and 130 define an angle α which is designated as 132. The geometryof the kneading paddle is constrained by the necessity of mating with asecond paddle whereby the tip of the first paddle wipes the second andthereby traces the required contour. Angle α 132, which defines theentrance angle to the passage past the tip, is severely limited by thisdesign constraint, and is not optimized for dispersion of material.

In contrast, the profile of a preferred embodiment of present invention100 is shown in FIG. 14. A dispersion disk 102 is illustrated in thebore 124 of a screw extruder 10. A line 134 is shown tangent to thedisk's surface near the tip 122. As before, a second line 136, tangentto the projected point on the barrel's inner surface 124, defines anangle α 138. Since the geometry of the dispersion disk 102 is notdictated by the necessity to trace the outline of an adjacent paddle,the angle α 138 can be much more acute and can be designed to produceexcellent dispersion of the material. It is anticipated that this angleα 138 will lie in the range of zero to 35°, while the typical angle α132 found in prior art kneading paddles from twin screw extruders is 35°to 50°. By way of comparison, this corresponding approach angle α inprior art single screw extruders is typically 90°, since there is noprogressively narrowing passage provided.

While the material is forced all the way into a progressively narrowingpassage, and through it, dispersive mixing action will be efficient.However, the polymer melt will have a tendency to bypass the high stressregions and take the path of least resistance by flowing around them ascan be seen in FIG. 15A & B.

In FIG. 15A, a dispersion disk 102 is seen in profile as well as a smallportion of the barrel's inner surface 124. The high stress region 120 isshown between the disk tip 122 and the barrel's inner surface 124. FIG.15B shows a front plan view of the same dispersion disk 102 with flow ofmaterial 140. Material 142 is shown flowing around the sides of the disktip 122. This will reduce the efficiency of the dispersive mixingaction.

This problem can be greatly reduced by the addition of a barrier wall inthe high stress region to close off the bypass route. FIGS. 16A & B showanother preferred embodiment 150 of the present invention in which twobarrier walls 154 have been added. FIG. 16A shows a side plan view ofthe modified dispersion disk 152 with barriers 154 at the tips 122. FIG.16B shows a front plan view of the modified dispersion disk 152 withmaterial flow 140 which is channeled through the high stress region 120.The efficiency of the dispersive mixing is thus enhanced.

This modification can also be used with prior art kneading paddles toimprove performance as seen in FIGS. 17A & B and 18A & B. FIG. 17A & Bshow side and front plan views of a prior art kneading paddle withoutmodifications. FIG. 18A shows a side plan view of a modified kneadingpaddle 160 which has been improved by means of the present invention. Agroove 162, seen in dashed line in FIG. 18A, has been cut into thesurface of the paddle 160. FIG. 18B shows a front plan view of themodified paddle 160 with the groove 162 included. This groove 162 canchannel material into the region between the tip 164 and the barrel wall124. Although the paddle's shape is still largely dictated by thetracing of its companion paddle, the region in the groove 162 can beangled differently to improve material dispersion. This is expected toproduce improved performance, although it may not reach the efficiencyof the dispersion disks 102.

There is a common problem which exists in regard to mixing elementswhich have a neutral helix angle, such as both kneading paddles anddispersion disks. This problem is that there may be stagnation pointswhich exist at the transition between the different disks. Thiscondition can be corrected by the use of transition elements between thestraight disks. FIG. 19 illustrates one form that transitional elementsmight take. Dispersion disks 102 are interleaved with one or moretransition elements 166. In this version of the preferred embodiment,the transition elements are flighted elements with a helix angle betweenzero and 90°. These elements 166 may be basically shaped as twisteddispersion disks with the angle of twist equal to the stagger angle 109(see FIG. 11) of the dispersion disks 102. Transition elements flights168 are shown connecting the tips 122 of the disks 102. These elementsprovide a gradual transition from one dispersion disk to another and aidthe flow of material while still maintaining the dispersive mixingcapability. It is to be understood that these transition disks may alsobe used with kneading paddles of the prior art to improve material flow.

It should be further understood that the improvements of the presentinvention can be used in screw extruders that contain multiple screws.Twin-screw extruders can be co-rotating with screws rotating in the samedirection, or counter-rotating with screws rotating in the oppositedirection. There are also triple-screw extruders, quadruple-screwextruders, and screw extruders with ten screws arranged in a circularpattern. All varieties of screw extruders can benefit from improveddispersive mixing by using the preferred embodiments of the presentinvention which are presented above. Increasing the number of highstress regions by using flights with the profiles shown above, is atechnique that can be useful in any of multiple as well as single screwextruders. Therefore, in addition to the above mentioned examples,various other modifications and alterations of the inventivedevice/process 10 may be made without departing from the invention.Accordingly, the above disclosure is not to be considered as limitingand the appended claims are to be interpreted as encompassing the truespirit and the entire scope of the invention.

Industrial Applicability

The present screw extruder 10 is well suited generally for applicationin any mixing process where a solid or liquid ingredient needs to bemixed dispersively in a viscous fluid. This may be the dispersion ofsolid agglomerates in a viscous fluid or the dispersion of liquiddroplets in a viscous fluid. It is particularly well suited for use inmixing blends of polymers or for mixing additives to polymers prior toextrusion forming.

Applications in the polymer field include the dispersion of solidpigments into polymers for making colored plastic products. Particularlywhere uniformity of color is important, it is very advantageous that thecolor particles be well mixed dispersively and broken down into smalleragglomerates than may be possible through merely distributive mixing.

The present invention 10 can also be used to improve the dispersion ofincompatible polymer components into a polymer matrix to produce polymerblends and alloys. Good dispersive mixing can be important in obtaininguniform material properties such as tensile strength, durability, etc.Reinforcing fillers can be added to a polymer matrix to produceincreased stiffness with greater uniformity using the present invention10.

When manufacturing conductive or semi-conductive materials, thedispersion of conductive fillers in a polymer matrix is enhanced by useof the present invention 10. The dispersion of magnetic fillers inplastic magnets, and dispersion of solid fillers for increasedresistance to oxidation can both be improved when using the improveddispersive mixer 10. The present invention 10 is also useful in themanufacture of rubber adhesives.

The viscous fluid to be dispersively mixed does not have to be plasticor polymer based. It is possible to mix food products such as dough,mashed potatoes, cooking oil, a slurry of grapes or fruit concentrates,honey or peanut butter. It can also be petroleum products like oil orrocket fuel, etc. All of these materials may benefit from the improveddispersive mixing which is provided by the present invention 10.

Additionally, since the mixing elements of the present invention can bemade to be modular, it is possible to customize configurations foroptimum performance with a particular material. The improvements of thepresent invention 10 may thus also be incorporated into existing screwextruders at reduced cost. Particularly, improvements to prior artkneading paddles may be made by using conventional machining methods toinclude the improved dispersive groove 162 in existing machines forlittle cost. For even better performance, dispersion disks 102, whichmay be manufactured to the same diameter and standard shaft fittingdimensions as the prior art paddles, may replace the kneading paddles.

For the above, and other, reasons, it is expected that the screwextruder 10 of the present invention will have widespread industrialapplicability. Therefore, it is expected that the commercial utility ofthe present invention will be extensive and long lasting.

What is claimed is:
 1. A screw extruder comprising:a barrel, said barrelhaving a bore defining an inner surface; and at least one extruderscrew, positioned within said bore, each screw including a central shaftand at least one screw flight, each flight including a front pushingface and a rear face, each said front pushing face having a profilewhich interacts with said inner surface of said barrel to form aprogressively narrowing passage through which material is forced intomultiple regions of high elongational and shear stress.
 2. The screwextruder of claim 1 wherein:said screw flights are arranged in a helicalpattern around said central shaft.
 3. The screw extruder of claim 1wherein:said profiles of said front pushing faces are selected from thegroup consisting of elliptical sections, circular sections andtriangular sections.
 4. The screw extruder of claim 1 wherein:saidextruder screws are twin extruder screws.
 5. The screw extruder of claim4 wherein:said twin extruder screws rotate in counter directions.
 6. Thescrew extruder of claim 4 wherein:said twin extruder screws rotate inthe same direction.
 7. The screw extruder of claim 1 wherein:saidextruder screw further includes distributive mixing means.
 8. The screwextruder of claim 7 wherein:said distributive mixing means are selectedfrom the group consisting of slots in the screw flights, pins on thecentral shaft, secondary flights, secondary flights with interruptions,changes in channel depth and changes in the channel width.
 9. The screwextruder of claim 1 wherein:said extruder screws include modularsections which are arranged on a central shaft to provide variablemixing characteristics.
 10. The screw extruder of claim 1 wherein:saidscrew flights are symmetrically arranged about a central shaft tobalance deflection forces.
 11. The screw extruder of claim 10wherein:the number of screw flights is a variable selected from thegroup consisting of two, three, four, five, and six.
 12. The screwextruder of claim 1 wherein:said extruder screw further includesconveying/wiping flights.
 13. A screw extruder comprising:a barrel, saidbarrel having a bore defining an inner surface; and a single extruderscrew positioned within said bore, said screw including a central shaftand a plurality of dispersion disks, each dispersion disk furtherincluding a front pushing face and a rear face, each said front pushingface having a profile which interacts with said inner surface of saidbarrel to form a progressively narrowing passage through which materialis forced into multiple regions of high elongational and shear stress,said dispersion disk profile further having defined an approach angle αto said progressively narrowing passage, said angle α lying in the rangeof zero to 35 degrees.
 14. The screw extruder of claim 13 wherein:saidextruder screw further includes a plurality of transition elements. 15.The screw extruder of claim 14 wherein:said transition elements areinterleaved with said dispersion disks.
 16. The screw extruder of claim15 wherein:said transition elements are twisted dispersion disks withthe angle of twist equal to the stagger angle of the dispersion disks.17. The screw extruder of claim 13 wherein:said dispersion disks furtherinclude barrier walls to channel material flow into high stress regions.18. A screw extruder comprising:a barrel, said barrel having a boredefining an inner surface; and a single extruder screw, positionedwithin said bore, said screw including a central shaft, and a pluralityof kneading paddles, each kneading paddle further including a dispersiongroove which channels material into a region of high stress.
 19. Thescrew extruder of claim 18 wherein:said extruder screw further comprisesa plurality of transition elements.
 20. A screw extruder comprising:abarrel having a bore defining an inner surface; and at least oneextruder screw positioned within said bore, each screw including acentral shaft and at least one fluted mixing section, each fluted mixingsection having at least one inlet channel and at least one outletchannel separated by a plurality of barrier flights which act to forcematerial through regions of elongational and shear stress.
 21. A methodof extruding material from a screw extruder comprising the stepsof:providing a screw extruder including a barrel having input and outputends, a bore defining an inner surface, and an extrusion die at saidoutput end;at least one extruder screw positioned within said bore, eachscrew including a central shaft and at least one screw flight, eachflight including a front pushing face and a rear face, each frontpushing face having a profile which interacts with said inner surface ofsaid barrel to form a progressively narrowing passage through whichmaterial is forced into multiple regions of high elongational and shearstress; introducing extrusion material into said input end of saidbarrel; rotating said screws to mix said material dispersively anddistributively; conveying said material towards said extrusion die atsaid output end to be shaped; and, extruding said extrusion materialfrom said extrusion die.
 22. The method for extruding material from ascrew extruder of claim 21 wherein:said profiles of said front pushingface portions are selected from a group consisting of ellipticalsections, circular sections and triangular sections.
 23. A screwextruder comprising:a barrel having a bore defining an inner surface; atleast one extruder screw having a screw diameter D, said screw beingpositioned within said bore, each screw including a central shaft, amixing section having a mixing section length L, and at least one screwflight, each flight including a front pushing face, a rear face and aflight tip having a flight width w_(f), the profile of each said frontpushing face acting with said inner surface of said barrel to form aprogressively narrowing passage through which material is forced intomultiple regions of high elongational and shear stress; said screwflights being arranged in a helical pattern around said central shaftand having defined a helix angle φ; said screw extruder having a channeldepth H defined as the distance between said central shaft and said boreinner surface, a radial flight clearance δ defined as the distancebetween said flight tip and said bore inner surface, and a flightclearance/channel depth ratio δ/H; and for a range of δ/H of 0.05-0.5,mixing section length L lies in the range 1-20 times the screw diameterD, the helix angle φ lies in the range from -90° to -30° and +30° to+90°, and the flight tip width w_(f), lies in the range 0-0.5 times thechannel width.
 24. The screw extruder of claim 23 wherein:said screwextruder further has defined a ratio of radial flight clearance to screwdiameter δ/D, where said δ/D lies in the range of 0.005-0.250.